The present invention relates to a steam injector and a steam injector system adapted for a nuclear reactor or boiler having high large discharging pressure and large capacity to supply water to the reactor or boiler and particularly adapted for a reactor core cooling water supply apparatus operated at an isolation or an emergency of a light water reactor (LWR).
Steam injectors are utilized in prior art for steam locomotives or boilers and, for example, generally have structures shown in FIGS. 11 and 12.
Referring to FIG. 11, reference numeral 2 denotes a casing provided with a steam supply port 1. A steam jet nozzle 4 provided with a needle valve 3 is incorporated in the casing 2, and a water suction port 5 is disposed adjacent to the steam jet nozzle 4. On the downstream side, righthand side as viewed, of the steam jet nozzle 4 are disposed a steam-water mixing nozzle 6 and a pressure increasing diffuser 7, which are communicated with a discharge port 9 through a check valve 8. The steam-water mixing nozzle 6 includes a throat 10 to which is opened an overflow water drain port 12 which is communicated with an overflow water duct 11. The steam injector of such structure may be called "central steam jetting type steam injector".
In the steam injector of the structure described above, when the needle valve 3 is drawn off from the steam jet nozzle 4 by operating a handle 13 secured to the needle valve 3 and the steam supplied from the steam supply port 1 is thereby jetted through the steam jet nozzle 4, the steam is flown into the steam-water mixing nozzle 6 while condensing the steam by a low temperature water sucked through the suction port 5 and having a water temperature lower by about 70.degree. C. than a saturation temperature of the supplied steam. The steam-water mixture is then flown through the throat 10 as high velocity stream.
Referring to FIG. 12, showing another type of steam injector, partially broken away, disposed vertically, the steam injector has a primary liquid inlet nozzle 14 at the central portion of the injector, and a casing 16 having a steam inlet nozzle 15 surrounds the primary inlet nozzle 14. A secondary liquid inlet nozzle 17 is mounted to the side surface of the casing 16 and a diffuser 19 having a lower opened end 18 is also mounted to the casing 16, thereby forming a steam-water mixing chamber 20 below the front end of the primary inlet nozzle 14 and within the steam nozzle 15.
FIG. 13 shows a characteristic analysis model of the steam injector such as shown in FIGS. 11 and 12. The model utilizes an analysis model evidenced by a low pressure visualizing test and adapts a single dimensional steady-state model which is easily analized in its design. Setting conditions are as follows.
(a) Steam flow quantity and flow velocity at the steam nozzle outlet are based on the critical flow theory. PA1 (b) Pressure, temperature and flow velocity at the throat are calculated in accordance with momentum and energy balance of the steam and water between cross sectional areas O and T. PA1 (c) Pressure is increased by the diffuser on the basis of the Bernoulli's theorem. PA1 (d) Experimental values are used for nozzle losses of respective portions.
Further, in FIG. 13, the respective letters or symbols denote as follows: A:area; m:mass flow rate; G: mass velocity (mass flux=m/A); .rho.:density; u:flow velocity; P:pressure; .zeta.:loss coefficient; and suffixes S, W, T, D and N represent the supply steam, the suction water, the throat, the diffuser and the nozzle, respectively.
Under the described set conditions, the steam flow rate m.sub.g and the steam flow velocity at the nozzle outlet port u.sub.so are obtained in accordance with the critical flow theory. Particularly, it is important that enthalpy of the steam h.sub.G is higher than that h.sub.L of the saturation water by latent heat of vaporization, and an amount corresponding to the difference between these enthaplies heat drop) is converted into kinetic energy of the steam from which loss in the nozzle is subtracted, thereby forming a steam supersonic flow. The suction water is accelerated by this steam supersonic flow from the sectional area 0 to the sectional area T of the main nozzle throat and the steam is condensed therebetween at the suction water surface to form condensed water. During this operation, the kinetic energy of the steam is transferred to the water, thereby forming the high velocity water flow. When the high velocity water flow passes the diffuser for increasing the pressure, an amount of pressure changed from dynamic pressure to static pressure increases on the basis of the Bernoulli's theorem. In the evaluation of the energy transfer amount from the steam supersonic flow to the high speed water flow, it is necessary to estimate the pressure loss in the mixing nozzle and the main nozzle, but this estimation is very difficult, and it is rather easy to obtain the pressure loss by calculating the momentum balance a two inspection surfaces of the sectional areas O and T. Supposing that the steam supersonic flow having the flow rate u.sub.so and the suction water having the flow rate u.sub.wo are mated with each other at the sectional area 0 and completely mixed and condensed at the sectional area T, a discharge pressure P.sub.D is expressed by the following equation in the adaption of the Beroulli's equation considering the nozzle loss between the throat T and the diffuser outlet port D. ##EQU1##
It was confirmed that the discharge pressure P.sub.D calculated by the equation (1) well coincides with the experimental result shown in FIG. 14, and the charactersitic feature of the steam injector is shown in FIG. 14, from which it will be found that a discharge water having a high pressure higher than the pressure of the supply steam can be obtained by a static type mechanism including no movable portion.
In the structures of the steam injector of the prior art, which are partially illustrated in FIGS. 15 and 16, the water jet flow is formed at the central portion of the steam-water mixing nozzle S by the water nozzle W and the steam supersonic flow is formed at an annular portion between the water nozzle W and the steam-water mixing nozzle S. The steam injector of such structure is for example disclosed in the Japanese Patent Laid-open Publication No. 63-289300. According to this structure, the water jet flow is sustained outside by the annular steam supersonic flow and accelerated without contacting the mixing nozzle wall, so that the less amount of the flow motion loss is obtained and the drain pressure is hence increased. However, the evaluation equation of the discharge pressure is expressed by the same equation in the one dimensional steady-flow model and the equation (1) can thus be utilized. The reduction of the flow motion loss is treated with by making small pressure loss coefficient .zeta..sub.N in the mixing nozzle in the equation (1) to a proper value. The steam injector of this structure may be called "outer periphery steam jetting type steam injector" in comparison with the aforementioned type steam injector.
However, in the case of a steam injector adapted for a nuclear power plant in which a large quantity of steam, 15 ton/hr. for example, is injected, there is a tendency of decreasing the discharge pressure by reasons described hereinlater. Furthermore, in such steam injector, it is difficult to secure a large amount of steam source for experiment for developing a large-sized steam injector. For example, in the experiment in which the steam of the amount of 15 ton/hr. is treated with, a large-sized boiler having a power of about 18 MW will be required.
The reason of the tendency of decreasing the discharge pressure in the steam injector of large capacity is explained hereunder with reference to FIGS. 15 and 16, in which FIG. 15 shows the case of a steam injector of small capacity and FIG. 16 shows the case of a steam injector of large capacity. With reference to FIGS. 15 and 16, since the flow rate is increased in proportion to square of the dimension thereof, cross sectional area of the water jet jetted from the water nozzle W is made large in the case of FIG. 16 in comparison with the case of FIG. 15. However, the surface area of the water jet flow F contacting the water and the steam is decided by the sectional area and the length of the water jet flow F. Even if the steam injectors of both the cases are constructed to have similar figures to make square the dimension of the water jet flow of the steam injector of FIG. 16 to that of FIG. 15, it becomes hard to transfer the heat from the surface of the water jet flow contacting the steam to the central portion thereof as the steam injector becomes large. For this reason, the steam condensing efficiency is lowered and the discharge pressure will be hence decreased. As described, as the operation of the steam injector is itself influenced by the heat transfer phenomenon to the central portion from the surface of the water jet flow colliding and condensing, it is difficult to effectively condense the steam to the water as the steam injector becomes large in structure.
Further, in FIGS. 15 and 16, since the flow cross sectional areas Aw at the front ends of the water nozzles W are constant, it is difficult to change the supply water rate to the steam injector, thus providing such problem as that the amount of the water flow drained from the overflow port of the steam injector increases when the discharge flow rate is throttled. As the overflow water cannot be poured in a pressure vessel and must be drained in a suppression pool of drain pit of a reactor, thus being not economical, and accordingly, it is necessary to reduce the amount of the overflow water as small as possible.
Furthermore, in FIGS. 15 and 16, a symbol X represents a closed portion of the critical flow and the steam on the downstream side from this portion X becomes the supersonic flow. In the case of FIG. 15, the area Y represents a portion in which the supersonic flow of the steam accelerates the water jet, and in the case of the small capacity, almost water is accelerated, whereas in the case of FIG. 16, showing the case of the large capacity, the accelerating area of the steam influences on only the surface area of the portion Z.
In the prior art, for the same purpose of increasing the discharge pressure, there is provided a steam injector system in which a plurality of steam injectors are connected in series. In this steam injector system, the water supplied accelerated in a first stage steam injector is further accelerated in a second stage steam injector for achieving high discharge pressure.
However, in such conventional steam injector system in which a plurality of steam injectors are connected in series to obtain the high discharging pressure, when a relief valve disposed at the second stage overflow drain exhaust port is operated, the inner pressure of the second stage steam injector is made instable, providing a significant problem.